Reference is made to FIG. 1. Touch screen display systems 10 are commonly incorporated in various electronic devices (such as smart phones, handheld computing devices, point of sale devices, GPS devices, mobile media players, remote control devices and the like) and include a touch screen system configured to detect a user input (i.e., user touch or hover) and a video display system (formed by a display panel 14 and a display drive circuit 18) configured to display content from a received video signal. The touch screen system comprises a touch screen panel 12 and a control and sense circuit 16 coupled thereto. The video display system comprises a display panel 14 and a display drive circuit 18 coupled thereto. In a common configuration, the touch screen panel 12 is substantially transparent and is mounted on and over the display panel 14 in a manner such that the content displayed by the display panel is visible to the user through the touch screen panel. The touch screen panel 12 is, for example, of the capacitive sensing type, and the display panel is, for example, of liquid crystal diode (LCD) type. The configuration and operation of the capacitive touch screen system to sense user input through the touch screen panel 12 to provide image data for display to the user through the display panel 14 are well known to those skilled in the art.
Reference is now made to FIG. 2. The touch screen panel 12 is typically configured to include an active region formed by a plurality of electrodes arranged in a sensor pattern. A first set of electrodes are formed by a plurality of parallel conductive lines 20 that are oriented to extend in a first direction (for example, horizontally) within the active region. A second set of electrodes are formed by a plurality of parallel conductive lines 22 that are oriented to extend in a second direction (for example, vertically) within the active region. In one known construction, the conductive lines 20 are formed by patterning a first layer of indium tin oxide (ITO) and the conductive lines 22 are formed by patterning a second layer of ITO. The first and second ITO layers are separated from each other by a layer of an insulating material (such as a dielectric material).
The use of perpendicularly oriented lines 20 and 22 represents just one of many known configurations for capacitive touch screen panel design. For example, the lines 20 and 20 could each comprise a set of series connected geometric shapes (diamond shapes are common). Still further, the lines 20 and 22 could instead be implemented to connect to individual pads arranged in an array format.
It is known in the art to configure the control and sense circuit 16 and touch screen panel 12 to operate in a mutual-capacitance mode. Reference is now made to FIG. 3. A capacitive sensing node 36 is formed at each intersection of the conductive lines 20 and 22 (separated by a dielectric material), and the capacitance of each sensing node 36 varies dependent on the proximate presence of the user (for example, the user's finger or an instrument such as a stylus under the control of the user). The control and sense circuit 16 includes a plurality of drive circuits 30 wherein an output of each drive circuit is coupled to a corresponding one of the conductive lines 20 of the panel 12. The control and sense circuit 16 further includes a plurality of sense circuits 32 wherein an input of each sense circuit is coupled to a corresponding one of the conductive lines 22 of the panel 12. Although an individual sense circuit 32 is illustrated for each conductive line 22, it will be understood that because of panel 12 size the sense circuits 32 may need to be shared amongst plural lines 22. The control and sense circuit 16 still further includes a controller 34 having control outputs coupled to the inputs of the drive circuits 30 and sense inputs coupled to the outputs of the sense circuits 32. In the mutual-capacitance mode of operation, the controller 34 sequentially actuates the drive circuits 30, with each actuation causing the application of an AC drive signal to the corresponding conductive line 20. The controller 34 then, for each drive circuit 30 actuation, actuates the sense circuits 32 (either in parallel or in sequence, and perhaps multiplexed) to sense the AC signal which is coupled to the conductive lines 22 through the capacitive sensing node 36. After completing a sensing scan as to all sensing nodes 36, the controller 34 processes the AC signals corresponding to each sensing node 36 that are sensed by the sensing nodes to make a detection of a user touch or hover and identify the location (coordinates) on the surface area of the panel 12 of that detection for output.
It is also known in the art to configure the control and sense circuit 16 and touch screen panel 12 to operate in a self-capacitance mode. Reference is now made to FIG. 4. Each conductive line 20 and 22 exhibits a capacitance with respect to a reference (such as ground) for the panel 12, and this line capacitance varies dependent on the proximate presence of the user (for example, the user's finger or an instrument such as a stylus under the control of the user). The control and sense circuit 16 includes a plurality of drive circuits 40 wherein an output of each drive circuit is coupled to a corresponding one of the conductive lines 20 and 22 of the panel 12. The control and sense circuit 16 further includes a plurality of sense circuits 42 wherein an input of each sense circuit is coupled to a corresponding one of the conductive lines 20 and 22 of the panel 12. Although an individual drive circuit 40 and sense circuit 42 are illustrated for each conductive line 20 and 22, it will be understood that because of panel 12 size the drive circuits 40 and sense circuits 42 may need to be shared (i.e., multiplexed) amongst plural lines 20 and 22. The control and sense circuit 16 still further includes a controller 34 having control outputs coupled to the inputs of the drive circuits 40 and sense inputs coupled to the outputs of the sense circuits 42. In the self-capacitance mode of operation, the controller 34 actuates the drive circuits 40 concurrently to apply a drive signal on the conductive lines 20 or 22. The sense circuit 42 that is coupled to each conductive line 20 or 22 operates to sense the line voltage or current in response to the applied drive signal. After completing a scan as to all conductive lines 20 and 22, the controller 34 processes the sensed line information to make a detection of a user touch or hover and identify the location (coordinates) on the surface area of the panel 12 of that detection for output.
As noted above, the panel 12 need not be configured in the form of perpendicularly intersecting lines (as shown in FIG. 2). This is especially the case when considering a touch system configured for operation in the self-capacitance mode. In such a case, the lines 20 and 22 may terminate at individual pads arranged in an array format as generated shown at references 120 and 122 in FIG. 4.
Reference is now made to FIG. 5. A video display system includes the display panel 14 (for example of the LCD-type) and the display drive circuit 18. The display panel 14 typically includes a thin film transistor (TFT) pixel array. The display drive circuit 18 includes a gate driver 50 and a source driver 52 coupled to and controlling the operation of the thin film transistors of the panel 14. A display controller 54, which receives an input video signal, generates gate control signals (G) to control operation of the gate driver 50 to generate gate drive signals applied to the gates of the thin film transistors (for turning on/off the pixels) and further generates source control signals (S) to control operation of the source driver 52 to generate source drive signals applied to the source terminals of the thin film transistors (for controlling pixel brightness). The display controller 54 further generates a common voltage signal (Vcom) for controlling the inversion of liquid crystal in the panel 14.
The received video signal comprises a vertical synchronization signal (Vsync), a horizontal synchronization signal (Hsync), and video information in the form of a red signal, a green signal and a blue signal. The time to display a single scan line by the panel is the time period associated with the horizontal synchronization signal. The time to display a single frame by the panel is the time period associated with the vertical synchronization signal. The video information (red/blue/green) is interpreted by the display controller 54 to determine which thin film transistors are to be actuated and at what brightness. The gate drive signals are sequentially asserted in response to the horizontal synchronization signal to sequentially actuate the scan lines of the panel. The timing relationship between several signals for display panel operation is shown in FIG. 6. The period during which the Vsync signal is low is referred to as the vertical blanking interval (VBI).
It is noted by those skilled in the art that the operation of the touch screen panel can have an adverse effect on the operation of the attached video display panel as the control signals generated for touch screen panel operation can induce perturbations, such as flicker, in the displayed image. There is a need in the art for solutions to this problem.